Modulation system



1940; G. LyoAvl es ET AL 2,198,025

ubnunnlou sYs'rEu Filed Dec. 2, 1937 1 5 J 9* B 2 Iv LOAD INVENTOR 60155, .DAVI

Patented Apr. 23, 1940 ten STATES PATENT. OFFICE MODULATION SYSTEM GomerL. Davies, Woodside, Md., and Frank G. Kear, Minersville, Pa., assignorsto Washington- Institute of Technology,

Inc., Washington,

D. 0., a corporation of Delaware Application December 2, 1937, SerialNo. 177,782

8 Claims.

modulation of radio frequency energy in general, it is particularlyintended to be applied to the modulation of energy transmitter atultra-' high frequencies.

It has heretofore been proposed to effect the modulation or suppressionof a carrier wave by mechanical means and, as known to the prior art,such mechanical means may employ variable capacitative or inductivecoupling means for varying the amplitude at frequency of the carrierwave in order to impress thereon the desired modulation. Such mechanicalmodulator means, while producing satisfactory modulation or carriersuspension, have the disadvantage of being expensive and bulky andrequiring a large number of moving parts. These devices have theadvantage, however, of permitting the degree and frequency of modulationto be varied by simple means, which advantage is not inherent inelectrical modulating means.

It is an object of this invention to provide a method and apparatus formodulating a carrier wave to any degree, which method and apparatus willretain the recognized advantages of so-called mechanical modulators butwhich will be more simple in construction and operation.

An object of the invention is to provide means for impressing modulationof any desired degree or type on the output wave of a source of radiofrequency energy, in the operation of which the current supplied by thesource will be shortcircuited through a network or supplied to the loadby reason of the variation of the impedance of the transmission line atthe point where the network is connected to the line.

A further object is to provide a method and means for modulating'radiofrequency energy, in which the transmission line connecting the sourceto the load is alternately connected to the load or short-circuited, inaccordance with the position of a variable coupling device connected tothe transmission line through a line of predeten mined length.

A further object is to provide a method of modulating the energy outputof a source of radio frequency energy,in which variable capacity meansare connected to the transmission line connecting the source to the loadat any point which is so separated from the output of the source thatthe source will be connected to the load when the capacity means is in aposition of minimum capacitance, and will be disconnected from the loadand itself unloaded' when the capacity .means is in a position ofmaximum capacitance.

Other objects and features of novelty will be apparent from thefollowing description and the annexed drawing, it being clearlyunderstood, however, that the invention is not limited in any way bysuch description and drawing, or otherwise than by the appended claims.

Referring to the drawing, in which similar reference characters refer tolike parts:

Fig. 1 is a circuit diagram illustrating a modulating system accordingto the invention;

Fig. 2 is a circuit diagram illustrating a secand form of the invention,and' Fig. 3 is a circuit diagram of a beacon system employing modulatingmeans according to the present invention.

In carrying out the invention, we propose to modulate the output of asource of radio frequency energy by short-circuiting the transmissionline which connects the source and the load, this being done at such arate and in such a degree as to provide the desired modulation, the

short-circuiting of the transmission line being eiie'cted at apoint onthe line which is so separated from the source that the source will notbe loaded when the line is short-circuited. This is efiected byshort-circuiting the line at any point on the line which is separatedfrom the source by any odd number of quarter wavelengths of the outputwave of the source.- It has been found that the termination of thetransmission line, as by short-circuiting, at any odd quarter wavelengthpoint along the line will effectively remove current from the load andwill, at the same time, prevent the overloading of the source during theperiod of short-circuiting.

A circuit for effecting modulation by the means according to thisinvention is disclosed in Fig. 1

of' the drawing. In this figure, i represents a source of radiofrequency current which is connected at point A to a transmissionline'2, to the end of which is connected a load 3 which may be anantenna or other load to which modulated energy is to be supplied andwhich is made equal to the surge impedance of the line.

In carrying out our invention we propose to short-circuit thetransmission line'2 at any point B along the length thereof which isspaced from the input 'end A of the transmission line by any quarterwave length of the energy output of the source. The short-circuiting ofthe line is effected by connecting to the transmission line at any pointB along the length thereof a variable capacity device 4, the terminals Cof which are connected to the point B of the transmission line by a line5, the length of which is equal to any even or odd quarter wavelength ofthe output wave of the source, or is equal to zero, the length of theline 5 including the loading effect of the variable capacity device 4.The capacity device 4 is preferably a motor-driven rotary condenser, theplates of which may be suitably shaped to provide any desired waveshape.

It will be seen that the operation of the variable capacity device 4will cause the transmission line 5 to be alternately opened andshortcircuited and that the impedance of the line 5 at the point C willbe reflected at the input end of this line, i. e. at point B, in amanner dependent upon the length of the line 5, this being due to thequarter-wave relationship between points B and C. The transmission line2 will therefore be alternately opened and short-circuited at point B.During periods when it is opened, i. e. when the variable capacitydevice A? is in its position of minimum capacitance, there will be noeffect on the transfer of power from the source to the load and duringsuch periods the full carrier output of the source is transmitted to theload. When the variable capacity device 4 is in position of maximumcapacitance and the line 2 is therefore short-circuited, no power willbe transferred to the load. Between these full-load and no-loadconditions, the current transmitted to the load will be dependent uponthe position of the rotating elements of the device 4 with respect tothe fixed element thereof and upon the shape of the plates of the device4. The wave shape of the modulated current at the load will'be dependentupon the shape of the plates of the capacity device 4, while thefrequency of modulation will be dependent upon the speed-of the rotatingelement of the capacity device. In another method of controlling themodulation, the degree of modulation may be controlled by regulating thecapacity of the condenser device 4 or by regulating ths ratio 01'maximum to minimum capacitance thereof.

It will be apparent that proper operation of the modulating systemdisclosed in Fig. 1 will be dependent upon the electrical lengths of thetransmission lines 2 and 5, and the range of reactance variation of thecapacity device 4. In order to improve the range of reactance variationof the capacity device 4, we propose to connect an additional circuitelement to transmission line 2 at the point B. In its preferred form,which is disclosed in Fig. 2 of the drawing, the additional elementtakes the formof a shortcircuited line 6 the terminals of which areconnected to transmission line 2 at point B. The electrical length ofthis short-circuited line is made such that the impedance across theline 2 at point B varies from zero. to infinity. It will be seen thatthe length of the line 8 may be adjusted to provide the conditionshereinbefore stated as requisite to the proper operation of themodulating system.

The transmission line --5 is adjusted to 'such length that the reactanceof the capacity device 4 will be reflected, in a manner dependent upon Ithe length of the'line, at the input end of this line, i. e. the pointB. Thus, if the line 5 is equal in length to an even number of 'quarterwavelengths of the energy supplied by the source the total impedance atpoint B is a minimum when capacity device 4 is in a position of maximumcapacitance, while if the line 5 is equal in length to an odd number ofquarter wavelengths this condition obtains at B when the capacity device4 is in a position of minimum capacitance. When the impedance at B is aminimum there is a very high impedance looking into the line 2 from thesource and no energy will be transmitted from the source to the load.The shortcircuited line 6 is so adjusted that at the minimum capacitanceposition of capacity device 4, when line 5 is an even number of quarterwavelengths, or at the maximum capacity position thereof, when line 5isan odd number of quarter wavelengths, the impedance across line 2 atpoint B is a maximum, thus having no effect on the transmission of powerfrom the source to the load.

In the operation of this system it will be seen that when the capacitydevice 4 is in a position of minimum capacitance, when line 5 is an evennumber of quarter wavelengths, or in a position of maximum capacitance,when line 5 is an odd number of quarter wavelengths, there is a fairlyhigh capacitative reactance in line 5 when this is viewed from point Band, at the same time, when looking into the short-circuited line 5 fromthe same point there is an inductive reac-tance of equal magnitude.These reactances being in parallel, the resultant impedance across theline 2 is very high and there is, consequently, no effect on thetransfer of power along the line. When the capacity device is in aposition of maximum capacitance; when line 5 is an even number ofquarter wavelengths, or in a position of maximum capacitance, when line5 is an odd number of quarter wavelengths, a condition of seriesresonance obtains when line 5 is viewed from point B and consequentlythere is a-very low impedance at point B which amounts practically to ashort-circuit. At this time the condition of short-circuited line 6 isimmaterial. In view of the low impedance across line 5 at this time nocurrent willfiow from the source through the transmission line 2 to theload. In theory there will be no power loss in line 5 at this time,

but this condition probably cannot be achieved, due to imperfectelements and connections in the system.

Due to the fact that point B is' separated from the output of the sourceby any quarter wavelength of the output wave of the source, whenshort-circuiting occurs at B due to the condition of maximum capacitanceof the device 4, there is a very high impedance when looking into theline 2 from the source and consequently no power will be delivered fromthe source to the line 2.

In Fig. 3 of the drawing there is disclosed the adaptation of thepresent invention to the operation of a radio beacon of the equi-signaltype in which overlapping directional fields are radiated, the currentscausing the radiated fields being impressed with characteristicmodulations or signals which may be received to provide an indication ofposition with respect to the source of the radiated fields. in Fig. 3two antennas l0, II are connected in parallel to a source of radiofrequency energy I2 by transmission lines l3 and I4. To thesetransmission lines there are' connectedcapacity coupling devices l5, I6by transmission lines l1, l8, respectively, and short-circuited lines I9, 20. The lines I1, I 9 and I8, 20 are connected to the re- In thecircuit disclosed 'spective transmission lines l3 and M at points B,both of which points are spaced from the output terminals of the sourcei2 by any odd number of quarter wavelengths of the energy output of thesource. The disclosed system constitutes, in efiect, a duplication ofthe system disclosed in Fig. 2 of the drawing and hereinbeforedescribed.

The variable capacity devices l5, it of the beacon system of Fig. 3 maybe so constructed and driven at such speeds as to provide any desiredmodulation of the energy supplied by the source i2. In the usual type ofdouble-modulation beacon, the energy radiated from one antenna may begiven a characteristic modulation of .cycles, while that radiated fromthe sec ond antenna may be given a characteristic modulation of 86.7cycles, although any other desirable characteristic modulations may beimpressed on the radiated carriers. Regardless'of the specific nature ofthe modulations desired, the capacity devices l5, It may be shaped andconstructed and driven at such speeds as to provide the desired result.all in the manner hereinbe fore described.

While we have illustrated and described various forms and embodiments ofour invention, it will be apparent to those skilled in the art thatother modifications, embodiments and improvements may be made, allwithout departing in any way from the spirit or scope of the inventionas disclosed herein, for the limits of which reference must be had tothe appended claims.

We claim:

1. In an electrical system comprising a source of radio frequencyenergy, a load, and a line connecting the source and the load, means formodulating the energy supplied by the source to the load and comprisingreactance means connected across the line at a point separated from theinput end of the line by any odd number of quarter wavelengths of thesupplied energy, and means for cyclically varying said reactance meansto cause the reactance at said point to vary from a condition ofparallel resonance to a condition of series resonance, to thereby causethe impedance across the line at said point to vary from infinity tozero.

2. In an electrical system comprising a source of radio frequencyenergy, a load, and a transmission line connecting the source and' theload, means for modulating the energy supplied by the source to the loadand comprising means connected across the line at a point separated fromthe input end of the line by any odd number of quarter wavelengths ofthe energy output of the source and operable to vary the impedance ofthe line at the point across connection thereto from zero to infinity,said means comprising a variable capacity device and a short-circuitedline connected in parallel and both connected to the line at said pointof connection, the short-circuited line having such an electrical lengththat it presents an inductive reactance across its ter- 3. In anelectrical system comprising a source of radio frequency energy, a load,and a transmission line connecting the source and the load, meansconnected in parallel and to the linebetween the source and the load andcomprising electrical means operable to vary from a capacitativereactance to series resonance when'viewed from the connection thereof tothe line, and a second electricalmeans connected to the line andoperable to provide an inductive reactance. whereby the impedance acrossthe line at the said point of connection varies from infinity to zero,to thereby modulate the energy supplied by the source to the load.

4. An electrical system comprising a source of radio frequency energy, aload, a transmission line connecting the source and the load, means formodulating the energy supplied by the source to the load comprising avariable capacity device connected across said transmission line by asecond transmission line having an electrical length of K degrees and ashort-circuited transmission line having an electrical length of Rdegrees, said second transmission line and said short-circuitedtransmission line being connected in parallel and both being connectedacross the first transmission line at said point of connection, K and Rbeing so chosen that the impedance across the first transmission line atsaid point of connection is caused to vary substantially from zero toinfinity as the capacity of said variable device is varied.

5. A modulating system comprising a source of radio frequency energy, aload, a transmission line connecting the source and the load, the loadbeing approximately equal to the surge imped ance of the transmissionline, a continuously variable capacity device connected across thetransmission line at a point spaced from the input end of the line byany odd number of quarter wave-lengths of the energy output of thesource,

' and means for cyclically varying the capacity of said variable deviceto continuously and progressively vary the impedance across thetransmission line at said point of connection substantially from zero toinfinity.

6. A modulating system according to claim 5, in which the variablecapacity device is connected to the transmission line through a secondtransmission line which is equal in length to any number of quarterwavelengths of the energy output of the source.

7. A modulating system according to claim '5 in which the variablecapacity device comprises a rotary capacitor having two fixed memberswhich are respectively connected to the transmission line members, and arotatable member which maybe moved into capacitative relation to eitherof said fixed members.

8. A modulating system according to claim 5, in which the variablecapacity device is connected directly across the transmission line.

